One type of atomic absorption spectrophotometers in which a plurality of light sources are used has, for example, a function for correcting background absorption with a hollow cathode lamp and deuterium lamp. In such an apparatus, a light beam provided from the hollow cathode lamp and a light beam provided from the deuterium lamp are combined by a beam combiner for example, pass through the space where an analysis sample solution is sprayed (globulized) and atomized, and then are introduced into a spectroscope to become a light beam of necessary wavelength range. After that, the light enters a photoelectric detector to be converted into an electric signal proportional to the light intensity. Furthermore, the electric signal is logarithmically converted. The light beam from the hollow cathode lamp is absorbed by the background and atoms to be analyzed, and the light beam from the deuterium lamp is absorbed by the background (where the absorption by atoms can be ignored because its narrow wavelength range). Since the signal which has been logarithmically converted is proportional to the intensity of absorption, the difference between the signals logarithmically converted from the electric signal proportional to the intensities of both light beams, i.e. amounts of light, is proportional to the intensity of the absorption by atoms, with the influence of the background absorption being eliminated.
In the meantime, a logarithmic conversion circuit for logarithmically converting a signal as previously described can appropriately operate only in a limited signal range. Hence, the conversion accuracy decreases in the case where the signal is excessively small. Given this factor, it is necessary to equalize, as much as possible, the intensity of the electric signal obtained by photoelectrically converting a light beam from the hollow cathode lamp and that of the electric signal obtained by photoelectrically converting a light beam from a deuterium lamp, which are provided into the logarithmic conversion circuit. Therefore, in a conventional apparatus, one of the following three manners is employed: (1) the gains of electric circuits corresponding to each lamp are adjusted in order to equalize the intensities of the electric signals corresponding to both lamps; (2) the electric power to be supplied to each lamp is adjusted in such a manner as to equalize the intensities of the electric signals corresponding to both lamps; and (3) a dimmer apparatus is provided for changing a transmissivity in a stepwise fashion between each lamp and a beam combiner in order to adjust the amount of light of both lamps to equalize the intensities of the electric signals.
If the amount of light from a hollow cathode lamp and that from a deuterium lamp are significantly different, the sensitivity of a photoelectric detector (e.g. a photomultiplier) is adjusted for the lamp having the larger amount of light. Accordingly, the signal to noise ratio (S/N) of the signal of the lamp having the smaller amount of light is deteriorated. Given this factor, a beam combiner has been proposed in which a light amount loss is reduced so that the amounts of light of a hollow cathode lamp and a deuterium lamp can be easily balanced (refer to Patent Document 1 for example).    [Patent Document 1] Japanese Unexamined Patent Application Publication No. S60-37518